The invention relates to an optical sensor for detecting the light intensity of combustion processes in a combustion chamber.
In combustion engines, for example in spark ignition engines or diesel engines, optical sensors are used to monitor the combustion. The temporal recording of the light intensity in a combustion chamber provides important data as boundary conditions for computer models, since one might use the light intensity to derive the temperature. The detection of the light intensity also correlates to the soot concentration in the combustion chamber. Because regulations of engine exhaust are becoming more and more restrictive, much effort is devoted to decrease soot formation during combustion in engines. Optical sensors provide valuable data for these developments.
Of specific interest are optical sensors exhibiting a detection angle of up to 130° and having such a small diameter that they might be installed in a sparkplug or a glow plug.
Test experiments revealed, that the side of the optical sensor facing the combustion chamber will be covered with soot after a certain period of time, in most cases after a few minutes. Thereby, the transparency of the front lens is reduced. After the soiling of said lens, the values measured by the optical sensor can be correlated to the light intensity in the combustion chamber only after a laborious computed calibration. For this purpose the sensor must be dismounted and cleaned after the calibration. This process disrupts the measurement, is very time consuming and expensive.
Experiences have shown that the soot may be burnt off and the front lens may be purified, when the temperature at said lens is at least 400° C. For this purpose in some applications, the front tip of the sensor is selectively exposed to the hot gas of the combustion chamber. Thereby, the soot is eventually burnt off again, depending on the temperature in the combustion chamber, and the measurement provides useful data again. In most cases this gas flow is unfortunately not sufficient to burn the front lens clean.